What does the Dock "A" Cable do?
 

I play a mechanical engineer on TV but this one has me baffled.

We are making an "A" to compensate for a tree that used to faithfully hold up our dock all winter that recently died. 80+ year old hemlock who's rings amazingly document the various growing conditions over the decades on the edge of the water.

Anyway, I have always been puzzled by the nearly universal practice of the incorporation of a compression cable you often see running below the center line of the A from top to bottom. Offset below about 1 foot, creating a bit of a suspension bridge.

I have struggled with various explanations for what the function of this stressing is for. Does it improve crumple resistance? Why does stressing the frame in one way do that? Does it help twist? Is the cable tight? How tight?

My most reasonable explanation...does it compensate for gravitational frame sag which might reduce compressive strength and initiate collapse?

Thanks for any help in quieting my obsession as I install one and waiting 6 months to see if I was successful.

It puts a tension member in the frame. If you pull up your dock without this line attached and site the A frame you will notice about 4-5inches of sag in the A frame. Then install the tension line and see that that sag reversed into a slight bow after tensioning it. Its sole purpose is to remove the sag, which could in fact cause the frame to buckle.

A structural truss always has two forces acting on it, compression and tension. Without that line everything is in compression and as an engineer, you know what happens when a balance of forces is not achieved.

The key element is that it is held away from the frame with a point member. Otherwise it is not doing anything.

A simple explanation: take a stick that is nice and straight, put your foot on one end and pull the other end toward you. The center of the stick bends away from you. Now try the same thing with a strung stick (like a Long Bow, http://en.wikipedia.org/wiki/File:Longbow.jpg with the center of the stick bent toward you), it will not react the same way and you will end up hurting yourself trying to bend it the wrong way. Add in the three point connection for rotational stability and you have the same thing as your dock support A frame.

Hope that helps.

RGuy,

My guess is that cable is not in compression, but rather has tension on it. It is under neath providing resistance to the top of the A frame as it pulls back. And as you mentioned is precisely for making sure the A frame itself doesn't buckle and fold back on itself. I would also guess that its importance is in direct relation to the Vertical stance of the a frame as it raises. More tension the closer to Horizontal it is, with less tension the closer to vertical it gets.

Does anyone have a picture?
 

I think a picture of what is being discussed would be infinitely useful in helping those of us who are having trouble understanding the physics involved here. Can anyone supply a picture of the A-frame and Cable?

http://www.docksource.com/wp-content...Appe-5-05C.jpg

The line being discussed can be seen just under the A frame.

I believe jmen24 is correct in that it reduces the sag. I think it also reduces any lateral (side to side) movement near the center of the A. The length and narrowness of the A makes it more like an H. The tension of the cable could reduce side to side the bending of the A legs ..... maybe.

It's obvious from the photo that jmen24 is correct.

The wire under the "A" frame is called a "Bobstay" and intended to prevent sag in the "A" Frame when it is near the horizontal position...when the dock is about to be raised. Technically, the bobstay wire keeps the plane of the "A" Frame "In Column". If the plane of the "A" Frame goes out of column, there is risk of buckling as mentioned above. If the "A" Frame were weightless, a bobstay would not be needed.

Iceboats have a "bobstay" ...with the little Strut just as in the dock "A" Frame, running fore to aft under the fusalage to keep it in column, as it resists down force from the mast and sail under load. BTW, if the "bobstay" is TOO tight, a compression fracture in the fusalage (the column) may result. ;) NB

PS: The picture was a big help in visualizing the situation. :)

The Physics of "Stays" vs. the Physics of "Mass"...
 

This concept of cable-physics is one that sailors know something about! ;)

http://www.liveantares.com/img/rigging.jpg

:eek:

But the physics of "stays" doesn't hold up well against the physics of Mass!

:eek: :eek: :eek:

http://latimesblogs.latimes.com/.a/6...f3ba970c-600wi

This boat was dismasted—nobody harmed. The whale swam off, apparently unharmed as well.

This genuine photo was accompanied by a genuine video from a different witness.

:offtopic:

Yes, yes...

I'm done.



:o